This application is based on Japanese Patent Application. No. HEI 9-191985 (filed on Jul. 2, 1997, for the invention made by Tabara and Nakaya), all the contents of which are incorporated herein by reference.
a) Field of the Invention
The present invention relates to a wiring forming method which is suitable to form a fine wiring in an LSI or the like, and more particularly to a wiring forming method aimed at improving the precision of the size of wiring patterns by forming antireflection coating on a wiring film and under a resist layer.
b) Description of the Related Art
A process for forming a wiring is indispensable for the manufacturing of a semiconductor integrated circuit. The wiring becomes complicated along with an improvement in the integration density, and the formation of a fine wiring and a multilayered wiring is required. After isolation regions and a large number of elements are formed in a semiconductor substrate, wiring for connecting those elements to each other is patterned. Wiring patterns are formed by depositing a wiring layer, forming resist patterns on the wiring layer and etching the wiring layer through utilization of the resist patterns as masks. If the base on which the wiring layer is formed is uneven, however, the surface of the wiring layer may also become uneven and have convex and concave parts (projections and recesses). Generally speaking, the wiring layer has a high reflectance with respect to light, especially with respect to short-wavelength light. When coating a resist layer on the uneven surface of the wiring layer and exposing the resist layer to light, the reflection of the light from the wiring layer is a problem.
A concave part of the surface of the wiring layer may form a concave mirror and the light reflected from the concave mirror may be converged at a region which is not to be exposed to light (this is known as xe2x80x9chalationxe2x80x9d). The halation causes the thinning and thickening of the wiring patterns, the breaking of the wiring and the formation of isolated spots.
A convex part of the surface of the wiring layer may form a convex mirror and the light reflected from the convex mirror may illuminate even a region which is not exposed to light. This degrades the accuracy of the light exposure.
The above-described phenomena can be reduced by reducing the light reflection from the underlying surface at the time of subjecting the resist layer to the light exposure.
It is generally known that in the case of forming a resist layer with the required patterns on a wiring material layer having a high reflectance by photolithography, antireflection coating is provided under the resist layer (and on the wiring material layer) so that the light reflection from the wiring material layer is suppressed to improve the pattern transfer accuracy. An inorganic single layer which is made of TION, TiN, SiON, SiN or the like is often employed as an antireflection coating film of this type. Sometimes an organic single layer which can be formed by a simple coating process is adopted (as seen from Published Unexamined Japanese Patent Applications Kokai Nos. 61-231182, 62-62523 and 62-63427, for example).
In the case of employing a TION (or TiN) single layer film as the antireflection coating film, the effect of preventing the reflection of a KrF excimer laser beam (having a wavelength of 248 nm) used in the far ultraviolet ray exposure is not satisfactory.
FIG. 13 shows the dependence of the reflectance on the film thickness. This dependence was obtained by performing a computer simulation as regards a TiON film P provided on a WSi2 (tungsten silicide) layer.
Reflectivity of a multi-layer structure was obtained by computer simulation under the following conditions.
On a substrate, m layers are stacked. The uppermost layer exposed to air (no=1, ko=0) is called the first layer. The underlying layers are called the second, third, . . . , and m-th layers. The substrate is called the (m+1)-th layer. The real part and the imaginary part of the complex refractive index xc3x1i of the i-th layer are denoted ni and ki. Therefore, xc3x1i=nixe2x88x92iki. The complex reflectivity is denoted by r, and the complex transmissivity is denoted by t. Complex reflectivities at the uppermost surface, the first, second, third, . . . , interfaces are denoted by ro, r1, r2, r3, . . . The complex reflectivity on the substrate surface is rm. Complex transmissivity at the first, second, third, . . . interfaces are denoted by t1, t2, t3, . . . The complex transmissivity at the substrate surfaces is tm. These notations are shown in FIG. 14.
The intensity reflection on the substrate surface Rm is             R      m        =                            "LeftBracketingBar"                      r            m                    "RightBracketingBar"                2            =                        "LeftBracketingBar"                                    (                              1                -                                                      n                    ~                                                        m                    +                    1                                                              )                        /                          (                              1                +                                                      n                    ~                                                        m                    +                    1                                                              )                                "RightBracketingBar"                2                        The  complex  reflectivity  of  the        ⁢          xe2x80x83        ⁢    j    ⁢          -th  layer        ⁢          xe2x80x83        ⁢          r              j        -        1              ⁢          xe2x80x83        ⁢          is                                                r                          j              -              1                                =                      xe2x80x83                    ⁢                                    [                                                                    {                                          exp                      ⁡                                              (                                                                              -                            2                                                    ⁢                          i                          ⁢                                                      xe2x80x83                                                    ⁢                                                      φ                            j                                                                          )                                                              }                                    ⁢                                      (                                                                  F                        j                                            -                                              r                        j                                                              )                                                  -                                                      F                    j                                    ⁡                                      (                                          1                      -                                                                        F                          j                                                ⁢                                                  r                          j                                                                                      )                                                              ]                        /                                                                    xe2x80x83                    ⁢                                    [                                                                    F                    j                                    ⁢                                      {                                          exp                      ⁡                                              (                                                                              -                            2                                                    ⁢                          i                          ⁢                                                      xe2x80x83                                                    ⁢                                                      φ                            j                                                                          )                                                              }                                    ⁢                                      (                                                                  F                        j                                            -                                              r                        j                                                              )                                                  -                                  (                                      1                    -                                                                  F                        j                                            ⁢                                              r                        j                                                                              )                                            ]                        ,                                where                    F        j            =                        (                                    n              o                        -                                          n                ~                            j                                )                /                  (                                    n              o                        +                                          n                ~                            j                                )                      ,          
        ⁢                  n        o            =      1        ,          
        ⁢                  φ        j            =              2        ⁢        π        ⁢                  xe2x80x83                ⁢                              n            ~                    j                ⁢                  d          /          λ                      ,          
        ⁢          λ      ⁢              :            ⁢              xe2x80x83            ⁢      wavelength        ,          xe2x80x83        ⁢    and        d    ⁢          :        ⁢          xe2x80x83        ⁢    thickness    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    the    ⁢          xe2x80x83        ⁢          layer      .      
The simulation adopted obtains rmxe2x88x921 by substituting rm, then rmxe2x88x922 by substituting rmxe2x88x921, . . . and ro by substituting r1.
The intensity reflection becomes
Ri=|ri|2.
The simulation conditions in that case were as follows:
Wavelength of light: 248 nm
Refractive index xe2x80x9cnxe2x80x9d and extinction coefficient xe2x80x9ckxe2x80x9d of TiON film:
n=2.28
k=1.5
Refractive index xe2x80x9cnxe2x80x9d and extinction coefficient xe2x80x9ckxe2x80x9d of WSi2 layer:
n=2.5
k=3.15
Reflectance at TiON/WSi2 interface: 54.9%
It can be understood from FIG. 13 that even though the film thickness was set at the optimum value, the reflectance could only be reduced to approximately 30% and thus the effect of preventing the light reflection was not satisfactory.
In the case of employing an SION (or SiN) single layer film as the antireflection coating film, a CVD (Chemical Vapor Deposition) apparatus is required for the film formation, which lacks simplicity. If a film having an ideal refractive index and extinction coefficient is intended, the realization of both the uniformity of the film thickness and throughput is difficult.
In the case of using an organic single layer film as the antireflection coating film, the precision of the size of the wiring patterns is low.
The organic antireflection coating film is made of an organic material of the same kind as a resist. An etching gas which contains oxygen as the main component is frequently used in the dry etching of the organic film. When the organic antireflection coating film is subjected to the anisotropic dry etching process using the resist patterns as masks after the formation of the resist layer, not only the antireflection coating film but also the resist layer is etched. In a film thickness range B shown in FIG. 12, the antireflection coating film is thick, and accordingly the time required for the etching is long. Due to this, the amount of shift in the size of the resist layer (the amount of thinning) is large, resulting in the degraded precision of the size of the wiring patterns.
FIG. 12 shows the dependence of the reflectance on the film thickness. This dependence was obtained by performing a computer simulation as regards an organic antireflection coating film Q provided on an WSi2 layer. The organic antireflection coating film Q may be formed of acrylic acid resin having side chains which contain organic group effectively absorbing KrF excimer laser light of a main wavelength of 248 nm, for example: 
where R is a portion absorbing light of a wavelength 248 nm, such as 
x=10 mol % to 80 mol %, and
y=20 mol % to 90 mol %.
Computer simulation was done using the formulae as described above. The simulation conditions in that case were as follows:
Wavelength of light: 248 nm
Refractive index xe2x80x9cnxe2x80x9d and extinction coefficient xe2x80x9ckxe2x80x9d of film Q:
n=1.654
k=0.23
Refractive index xe2x80x9cnxe2x80x9d and extinction coefficient xe2x80x9ckxe2x80x9d of WSi2 layer:
n=2.5
k=3.15
Reflectance at film Q/WSi2 interface: 54.9%
It can be understood from FIG. 12 that in a film thickness range A, for example, reflectance variations versus film thickness variations are considerable. Normally the surface on which a wiring is to be formed is uneven and has convex and concave parts, and an organic antireflection coating film is formed on such a surface by a spin coating method or the like. A portion of the antireflection coating film which is located on the top of a convex part and another portion of the antireflection coating film which is located on the bottom of a concave part differ considerably in thickness from each other. In such a case, when the organic antireflection coating film is formed within a film thickness range like the range A, the reflectance variations are so large that the accuracy of the transfer of fine patterns is degraded. In consideration of this, the organic antireflection coating film is formed within a film thickness range such as the range B in which the reflectance variations are small. In the film thickness range B, however, the antireflection coating film has a large thickness of approximately 100 nm.
When the organic antireflection coating film is formed on the uneven surface by the spin coating method or the like, a portion of the antireflection coating film which is located on the top of a convex part of the surface and another portion of the antireflection coating film which is located on the bottom of a concave part of the surface differ considerably from each other. In order to completely remove the antireflection coating film from the top of the convex or higher level part and the bottom of the concave or lower level part by subjecting the antireflection coating film to the anisotropic dry etching process which uses the resist layer as a mask, over-etching has to be performed even after the wiring material layer appears at the top of the convex or upper level part where the antireflection coating film is relatively thin and until the wiring material layer appears also at the bottom of the concave or lower level part where the antireflection coating film is relatively thick. Due to this, the amount of shift in the size of the resist layer is increased, degrading the precision of the size of the wiring patterns.
It is accordingly an object of the present invention to provide a wiring forming method which can improve the precision of the size of the wiring patterns.
According to one aspect of the present invention, there is provided a wiring forming method comprising the steps of:forming a wiring material layer on an insulation film covering one of major surfaces of a substrate; forming a first antireflection coating film made of one of TiON and TiN on the wiring material layer; stacking a second antireflection coating film made of an organic material directly on the first antireflection coating film; coating a resist layer on a lamination film which includes the first and second antireflection coating films, and exposing the resist layer to light in accordance with predetermined wiring patterns; forming resist patterns by developing the resist layer which has been exposed to light; and selectively removing the second antireflection coating film by anisotropic dry etching process which uses the resist patterns as masks, in order to leave patterns of the second antireflection coating film which correspond to the resist patterns.
Since the first antireflection coating film which is made of TiON or TiN is provided under the second antireflection coating film which is made of an organic material, the thickness of the second antireflection film can be reduced. A reduction in the thickness of the second antireflection film results in a reduction in the time required for performing the dry etching of the second antireflection coating films through utilization of the resist patterns as masks. Accordingly, the amount of shift; in the size of the resist patterns is reduced such that the precision of the size of the wiring patterns is improved.
The first antireflection coating film and the wiring material layer formed thereunder can be selectively etched using the resist patterns and the patterns of the second antireflection coating film as masks.
In order to form patterns of the first antireflection coating film, the first antireflection coating film may be selectively removed by the anisotropic dry etching process which uses the resist patterns and the patterns of the second antireflection coating films as masks, and then the resist patterns and the patterns of the second antireflection coating film may be removed. The patterns of the first antireflection coating film can be used as masks in a later etching process.
Such a thin resist layer as can serve only as a mask at the time of etching the first and second antireflection coating films will suffice. The use of the thin resist layer ensures an improved definition in transferring the wiring patterns to the resist layer and permits the depth of focus to be greater than the thickness of the resist layer so that fine wiring patterns can be transferred with high accuracy to the resist layer.
Thus, a lamination film including a TiON (or TiN) film and an organic antireflection coating film stacked thereon is used as the antireflection coating provided under the resist layer. This permits the organic antireflection coating film to be formed thin so that the amount of shift in the size of the resist layer at the time of etching the organic antireflection coating film is reduced to improve the wiring patterning accuracy.
After the removal of the resist layer and the patterns of the organic antireflection coating film, the patterns of the TiON (or TiN) film can be used as masks in etching the wiring material layer. This allows the resist layer to be formed thin so that the accuracy of the transfer of fine wiring patterns to the resist layer is improved to increase the yield of the wiring formation.